Nearly a year after embarking on a unique week-long tour via railroad from San Francisco to New Orleans, Edward Sharpe and the Magnetic Zeroes, Mumford and Sons and Old Crow Medicine Show will celebrate the premiere screening of a documentary about the trek with a massive free show in Austin, Texas next week during the South By Southwest festival.

As was previously announced, Big Easy Express, a 67-minute film directed by Emmett Malloy, will have its world premiere during SXSW on Saturday, March 17th, followed by a brief acoustic set featuring members of the three bands. But today the bands’ fans received an alert about an additional screening of “Big Easy Express” and even bigger performance on the field of Austin High School, to be presented by MySpace. Tickets will be distributed to fans who RSVP via MySpace.com/BigEasyExpress. MySpace will webcast the concert live and offer an HD version the following day.

“The memories from that tour are tattooed on my mind,” Magnetic Zeroes multi-instrumentalist Nora Kirkpatrick tells Rolling Stone. “When I first saw the finished film, it reminded me of what radiant moments live performances can create, and how powerful a group of minds can be when focused on the same goal.”

Last spring, I traveled along with the six-city “Railroad Revival Tour” all the way from San Francisco to New Orleans. Though close to 32,000 concertgoers attended the six shows along the way, the documentary will let viewers see for the first time how much music the bands made on board the fifteen vintage rail cars that carried them for the week. As stirring and impressive as the concerts were, the real magic happened on that train, with the musicians and assorted friends jamming until dawn every night. It was a truly remarkable thing to witness: Mumfords and Zeroes and Crows would pick up instruments and launch into extended improvisations ranging from low-key folk explorations to horn-led funk interludes to bluegrass sessions with several guitars, banjos, mandolins and percussion going at once.

“I’ll bet this picture helps us sell another 10,000 banjos,” says Old Crow Medicine Show frontman Ketch Secor. “Not to mention what it’ll do for Amtrak. Like the song says, “John Henry was a steal-driving man” – well he was, it’s true – and somewhere in some movie theatre of all places, somebody’s going to hear his hammer ring for the very first time. There’s not enough movies today that make you feel like that, that make you reconnect with something inside yourself that was always there, that old familiar voice of American song.”

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Director Malloy, who also filmed the 2010 White Stripes tour documentary Under Great White Northern Lights, says he remembers the final night of the tour with special fondness: Most of his crew had fallen asleep, but he kept shooting as Magnetic Zeroes frontman Alex Ebert led a group of musicians through a new song he’d started earlier that day. “What we got that night was pretty magical and ended up being the final scene of the film,” he says.

“It’s not just the music,” Mumford and Sons’ Ben Lovett told Rolling Stone while we were still out riding the rails. “Everyone on that train has something that they put into the melting pot of this really complicated but beautifully balanced concoction. There’s no feeling of segregation between the musicians and the friends and everyone who’s doing everything else on the train. It doesn’t feel like three bands separately furthering their audience fanbases or any of that shit. It’s like, everything’s taking a backseat and everyone on that train has mucked in to make it what it is.”